In medical treatment, such as dental treatment, operations requiring high precision are carried out where human errors may be harmful or even dangerous to a patient. In this respect, a standard general illumination system is not necessarily an optimal solution to enable precision work, but the area to be operated on, such as an oral cavity, is typically illuminated using separate operation lights. The operation light may be arranged to be adapted for use in connection with a particular operation or diagnosis and/or to have some functions adjustable e.g. on patient-specific basis. The light produced by the operation light must also be bright enough in order to allow for the operation to be carried out safely and effectively. However, the light must not be too bright so that it would dazzle the person performing the operation or the patient. Also, the general illumination of the operating environment has to be so implemented that no excessive contrast will be created between the area to be operated on and the operating environment.
In prior-art operation lights the light sources used include halogen bulbs and LED components. However, none of the manufacturers in the dental industry have yet brought any LED-based operation lights to the market. A problem with lights based on halogen bulbs is that they warm up intensively and may thus cause burns. In addition, halogen bulbs always involve a risk of explosion. This type of lights are also available provided with a fan, but typically the fan makes the light noisy, complicated in structure and unhygienic. A further problem with halogen bulb based lights is the relatively short life time of the bulbs, which involves extra service costs. Further, if brightness of a halogen bulb light is adjusted e.g. during an operation, this may have an undesirable consequence of change of color temperature as well. The holders (sockets) typically used with halogen bulbs heat up as well, which makes them unreliable components.
LED lights can be constructed to be fairly compact and light. Also, they require no mechanical components subject to wear, such as noisy fans. In addition, the electronics of a LED light can be arranged to be relatively simple and therefore inexpensive. It is also possible to provide LED components with an integrated reflector, in which case many light applications will not need a separate reflector or lens for directing light at all. Besides, the use of such lenses may produce a so-called rainbow effect at the edge of the light beam directed at the area to be operated on.
In general, a LED light only produces so-called cold light, because infrared radiation, i.e. radiation that generates heat, is typically very slight in the direction of the beam produced.
The maintenance costs of a LED light are also relatively small since the theoretical life time of LED components in continuous use is very long, even over 100 000 hours. Moreover, the LED light involves no risk of explosion, so it can be constructed without an explosion shield or other protective structures. As convectional cooling alone is sufficient, there is also no need for separate ventilation holes, which are unhygienic and get dirty.
Structurally, a LED component is a semiconductor junction and it is typically manufactured from gallium arsenide (GaAs), gallium arsenide phosphide (GaAsP), gallium phosphide (GaP) or some other corresponding material. The LED component is generally connected in the forward direction, because if connected in the reverse direction it will not produce any light and may even be damaged. The LED component is preferably fed by a supply voltage, which is equal to its threshold voltage, i.e. typically a voltage of about 1.1-3.8 V. If the LED component is fed by a voltage substantially higher than the threshold voltage, the supply voltage exceeding the threshold voltage is preferably passed e.g. to a series resistor in order to prevent damage to the LED component. The connecting lines of the LED component are the same as in an ordinary diode, i.e. anode and cathode.
Typically the operation of a LED component is based on charge carriers, i.e. electrons and holes, which move across a semiconductor junction, due to the effect of forward current, and emit photons upon recombination, i.e. upon being united again, which appears as emitted light. The color of the light emitted in the light emission process depends on the semiconductors forming the junction and the doping used in them. For example, gallium phosphide (GaP) doped with zinc (Zn) and oxygen (O) generally produces red light.
Typical standard LED components include red, yellow and greed LED components. Today, standard LED components are generally available in two sizes; in round packages of 3 and 5 mm in diameter. In addition, there are e.g. orange LED components whose packaging typically corresponds packaging of the standard LED components, and so-called transparent LED components, which have a clear package but the color of light is typically red, green or yellow, depending on the semiconductors of the component, or on the doping used in them.
An RGB LED component typically comprises, as indicated by its name, a Red, a Green and a Blue LED component. By means of an RGB LED component it is possible to produce any of these colors of the LED components and mixtures thereof, in fact any color within the color spectrum in question. The mixing of colors is typically accomplished by directing the light beams produced by the LED components to the same spot. In this case, however, it is necessary to take into account that different wavelengths undergo different refractions. For instance, blue light is refracted considerably more than red light.
There are also available e.g. LED components emitting white light. One possibility of implementing a LED component emitting white light is to integrate red, green and blue LED components with each other. However, in this case there is the problem that it is difficult to maintain a constant color temperature, because color temperatures of the LED components manufactured from different material mixtures change in different ways according to the temperature, the power supplied and the age of the component. Another possibility is to provide the LED component with fluorescing material absorbing the wavelength of the LED component used, and emitting a wavelength or wavelengths longer than absorbed, which fluorescing material may consist of e.g. different phosphors or phosphor layers. The LED component may also be composed of an ultraviolet LED component and phosphor. From the sum and combined effect of the different wavelengths produced it is possible to generate light of a substantially different color, e.g. white.
However, even white LED components exhibit a relatively wide variation of color temperature. For example, for a nominal color temperature of 5500K the variation of color temperature may be in the range of 4400-8000 K. This variation depends especially on the thickness of the phosphor layer deposited on the LED components during manufacture. To normalize the color temperature white LED components generally have to be measured to make it possible to select the ones having color temperature of e.g. about 5500 K. However, the variation of color temperature of white LED components means that even lights composed of several white LED components contain, precisely speaking, LED components emitting different colors.
Publication U.S. Pat. No. 6,459,919 discloses a general-purpose LED operation light and publication WO 02/06723 discloses a LED operation light applicable for dental use. The LED operation light described in WO 02/06723 publication generates a light field having a predetermined size, illumination intensity, uniformity of illuminance and color temperature. The first and second light fields consist of light beams generated by several LED components placed close to each other so that the second light field at least partially covers the first light field. According to the publication, brightness of the light field produced by the light can be influenced by implementing alignment of the individual light beams produced by the LED components arranged to it different, and the illumination intensity influenced by varying the number of LED components connected.
Thus, a typical problem with prior-art operation lights is that color temperature of the light produced by them does not remain essentially constant or as adjusted. If brightness of a halogen bulb light is adjusted during an operation, the color temperature of the light produced by it will change. On the other hand, the color temperature of LED operation lights may change e.g. with aging of the LED components producing different colors, because the mutual relations between the color components change as the light emissions produced by the LED components decay in different proportions. Color temperature here refers to the mutual ratio between the color components produced by the operation light. The color temperature of an operation light is typically adjusted to about 5000-6000 K, which corresponds to the luminosity of a cloudy midday.
In some operations it may also be advantageous to have a possibility to use a color temperature other than only a given predetermined color temperature. In prior-art operation lights there is no possibility to keep the color temperature within desired limits nor to adjust the color temperature as desired according to the needs of an individual operation on one hand, and so that it could be kept at a desired or constant setting throughout the life time of the LED operation light on the other.